1. Field of the Invention
The present invention relates to ceramic shell molds and, more particularly, to means for their manufacture, especially for the firing of ceramic molds at elevated temperatures.
2. Description of the Prior Art
In casting directionally solidified articles, either columnar grained as taught in the VerSnyder patent, U.S. Pat. No. 3,260,505, or single crystal as taught in the Piearcey patent, U.S. Pat. No. 3,494,709, it is common to solidify the molten metal in a casting assembly comprising an open-ended ceramic shell mold having a flat base positioned on a chill plate. The shell mold may be fabricated to produce a single cast article or a plurality of articles at one time and to this end includes one or more mold cavities in the shape of the article to be cast. Generally, the ceramic shell molds utilized have an open end above the mold cavity through which molten metal is introduced and an open end below the mold cavity through the mold base to allow contact between the molten metal and chill plate for unidirectional solidification. Of course, it is essential that mold base flatness be maintained to prevent leakage or runout of molten metal between the mold base and chill plate during casting.
Ceramic shell molds for use in these directional solidification processes can be formed by the lost wax process wherein a wax article pattern having a flat wax base is repeatedly dipped in ceramic slurry, dusted with ceramic stucco and dried until the desired thickness for a mold wall and base is obtained. After removal of the wax pattern and base, the green ceramic shell mold is fired at elevated temperatures, such as 1800.degree. F., to impart sufficient strength thereto to withstand casting stresses. However, the firing step has been found to be a source of mold base unevenness or irregularity. Usually, the green shell mold base is positioned on a metal furnace tray for firing, such furnace trays oftentimes having a nonflat surface as a result of warpage and thermal distortion from numerous mold firings. It has been discovered that when the green mold base is placed on an uneven furnace tray surface, the base tends to conform to the tray surface during firing, resulting in the observed unevenness which leads to molten metal runout. In the past, molds having such defective bases have been subjected to a further operation to provide the required mold base flatness, such operation including abrading the mold base with suitable grit paper until adequate flatness is achieved. In addition to being a time consuming, laborious process, sanding of the mold base is also disadvantageous since it introduces ceramic inclusions into the mold which inclusions oftentimes ultimately appear in articles cast in the molds. Of course, the presence of inclusions in the cast articles adversely affects their quality and mechanical properties and leads to their rejection.
Another problem associated with the prior art firing step is that hot spots develop in the green shell mold due to inadequate circulation of the furnace atmosphere therethrough as a result of the bottom mold opening being closed off by the furnace tray. These hot spots manifest themselves in the form of cracks, bulges and the like which defects are, of course, cause for mold rejection.
A further problem associated with the firing step is that loose ceramic particulate inside the firing furnace sometimes falls into the shell molds. As explained above, these inclusions can adversely affect the quality of the casting produced.